The present disclosure is related to marking and printing systems, and more specifically to variable data lithography system using fog development of an electrographic image for creating a fountain solution image.
Offset lithography is a common method of printing today. For the purpose hereof, the terms “printing” and “marking” are interchangeable. In a typical lithographic process a printing plate, which may be a flat plate, the surface of a cylinder, belt, and the like, is formed to have “image regions” formed of hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. The image regions are regions corresponding to the areas on the final print (i.e., the target substrate) that are occupied by a printing or a marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by the marking material.
The Variable Data Lithography (also referred to as Digital Lithography or Digital Offset) printing process usually begins with a fountain solution used to dampen a silicone imaging plate on an imaging drum. The fountain solution forms a film on the silicone plate that is on the order of about one (1) micron thick. The drum rotates to an ‘exposure’ station where a high power laser imager is used to remove the fountain solution at the locations where the image pixels are to be formed. This forms a fountain solution based ‘latent image’. The drum then further rotates to a ‘development’ station where lithographic-like ink is brought into contact with the fountain solution based ‘latent image’ and ink ‘develops’ onto the places where the laser has removed the fountain solution. The ink is usually hydrophobic for better adhesion on the plate and substrate. An ultra violet (UV) light may be applied so that photo-initiators in the ink may partially cure the ink to prepare it for high efficiency transfer to a print media such as paper. The drum then rotates to a transfer station where the ink is transferred to a printing medium such as paper. The silicone plate is compliant, so an offset blanket is not used to aid transfer. UV light may be applied to the paper with ink to fully cure the ink on the paper. The ink is on the order of one (1) micron pile height on the paper.
The formation of the image on the printing plate is usually done with imaging modules each using a linear output high power infrared (IR) laser to illuminate a digital light projector (DLP) multi-mirror array, also referred to as the “DMD” (Digital Micromirror Device). The mirror array is similar to what is commonly used in computer projectors and some televisions. The laser provides constant illumination to the mirror array. The mirror array deflects individual mirrors to form the pixels on the image plane to pixel-wise evaporate the fountain solution on the silicone plate to create a fountain solution image. If a pixel is not to be turned on, the mirrors for that pixel deflect such that the laser illumination for that pixel does not hit the silicone surface, but goes into a chilled light dump heat sink. A single laser and mirror array form an imaging module that provides imaging capability for approximately one (1) inch in the cross-process direction. Thus a single imaging module simultaneously images a one (1) inch by one (1) pixel line of the image for a given scan line. At the next scan line, the imaging module images the next one (1) inch by one (1) pixel line segment. By using several imaging modules, comprising several lasers and several mirror-arrays, butted together, imaging function for a very wide cross-process width is achieved.
Due to the need to evaporate the fountain solution, in the imaging module, power consumption of the laser accounts for the majority of total power consumption of the whole system. Such being the case, a variety of power and cost saving technologies for the imaging modules have been proposed. For example, the schemes to reduce the size of the image formed on the printing plate, changing the depth of the pixel, and substituting less powerful image creating source such as a conventional Raster Output Scanner (ROS). To evaporate a one (1) micron thick film of water, at process speed requirements of up to five meters per second (5 m/s), requires on the order of 100,000 times more power than a conventional xerographic ROS imager. In addition, cross-process width requirements are on the order of 36 inches, which makes the use of a scanning beam imager problematic. Thus a special imager design is required that reduces power consumption in a printing system. An overlooked area of power conservation is the use of non-laser imagers or alternative ways of creating the fountain solution image.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for increasing speed and lowering power consumption in variable data lithography system.